The present invention relates to a process for blending block copolymers of vinyl aromatic hydrocarbons and conjugated dienes with bitumen and crosslinking them with sulfur. More particularly, the present invention relates to such a process which utilizes high shear mixing.
Asphalt is a common material utilized for the preparation of paving and roofing materials and also for coatings such as pipe coatings and tank liners. While the material is suitable in many respects, it inherently is deficient in some physical properties which it would be highly desirable to improve. Efforts have been made in this direction by addition of certain conjugated diene rubbers, ethylene containing plastics like EVA and polyethylene, neoprene, resins, fillers and other materials for the modification of one or more of the physical properties of the asphalt. Each of these added materials modifies the asphalt in one respect or another but certain deficiencies can be noted in all modifiers proposed. For example, some of them have excellent weather resistance, sealing and bonding properties but are often deficient with respect to warm tack, modulus, hardness and other physical properties; and some of them improve only the high temperature performance of asphalt, some only improve the low temperature performance of asphalt, while some lack thermal stability or mixing stability with asphalt.
Since the late 1960s, diene polymer rubbers such as styrenebutadiene rubber and styrene-rubber block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers have been used to dramatically improve the thermal and mechanical properties of asphalts. Practical application of the rubber addition approach requires that the blended product retain improved properties and homogeneity during transportation, storage and processing. Long term performance of elastomer-modified asphalts also depends on the ability of the blend to maintain thermal and chemical stability.
Such bitumen/block copolymer mixtures have been crosslinked with a crosslinking agent such as sulfur to provide improved mechanical properties. A number of methods for incorporating the sulfur have been used. One of these involves mixing the bitumen and the copolymer together and agitating them for a period of time before adding the sulfur. After the sulfur is added, the agitation is continued for a period of time. This method is advantageous but has the disadvantage that the resulting morphology is relatively coarse. A very fine dispersion of the polymer would provide better high temperature properties. Therefore, it would be advantageous to provide a process for producing such bitumen/block copolymer blends which have a stable and very fine morphology. The present invention provides such a process.